Cell division and cell death play central roles in the proper development of multi-cellular organisms and in the homeostatic maintenance of tissues. Loss or reduction of cell proliferative capability and dysregulation of cell death are among the most important phenomena that characterize the aging process (D. Monti et al., Am. J. Clin. Nutr., 1992, 55 (6 Suppl): 1208S-1214S; H. R. Warner et al., J. Am. Geriatr. Soc., 1997, 45: 1140-1146; L. Ginaldi et al., Immunol. Res., 2000, 21: 31-38). Disruption of normal control of cell proliferation and cell death also underlies many pathological conditions including cancer; infectious diseases such as acquired immunodeficiency syndrome (J. C. Bentin et al., J. Clin. Immunol., 1989, 9: 159-168; R. A. Gruters et al., Eur. J. Immunol., 1990, 20: 1039-1044; L. Meyaard et al., Science, 1992, 257: 217-119; A. Cayota et al., Clin. Exp. Immunol., 1992, 88: 478-483); vascular disorders such as atherosclerosis and hypertension (S. M. Schwartz et al., Circ. Res., 1986, 58: 427-444; A. Rivard and V. Andres, Histol. Histopathol., 2000, 15: 557-571); and neurodegenerative diseases such as Alzheimer's disease (Z. Nagy, J. Neural Transm. Suppl., 1999, 57: 233-245; A. K. Raina et al., Prog. Cell Cycle Res., 2000, 4: 235-242; I. Vincent et al., Prog. Cell Cycle Res., 2003, 5: 31-41).
The most characteristic biochemical feature of cell division is DNA synthesis, which occurs essentially only during the S phase of the cell cycle (S. Sawada et al., Mutat. Res., 1995, 344: 109-116). Accordingly, the most commonly used methods for the study of cell cycle, DNA synthesis and cell proliferation rely on incorporation of labeled biosynthetic precursors into the newly synthesized DNA of proliferating cells (M. Bick and R. L. Davidson, Proc. Natl. Acad. Sci. USA, 1974, 71: 2082-2086; H. G. Gratzner, Science, 1982, 218: 474-475; F. M. Waldman et al., Mod. Pathol., 1991, 4: 718-722). In these methods, labeled DNA precursors (e.g., [3H]-thymidine or 5-bromo-2′-deoxyuridine (BrdU)) are added to cells during replication, and their incorporation into genomic DNA is quantified following incubation and sample preparation. Incorporated [3H]-thymidine is generally detected by autoradiography. Detection of incorporated BrdU is performed immunologically after sample denaturation to allow access of monoclonal antibodies, and the resulting BrdU-labeled cells are then analyzed by flow cytometry or microscopy. To study cellular proliferation of specific tissues, animals are administered (e.g., injected) labeled DNA precursors, sacrificed, and the tissues are removed and fixed for microscopic analysis.
Although [3H]-thymidine and BrdU incorporation labeling methods have proven valuable for studying cell cycle kinetics, DNA synthesis and sister chromatid exchange, as well as for assessing cell proliferation of normal or pathological cells or tissues under different conditions, these methods exhibit several limitations. The most notable disadvantage of [3H]-thymidine incorporation results from the complications and risks of using radioactivity. In addition, autoradiography is labor-intensive and time-consuming. Furthermore, because both methods are sample destructive, quantification can be performed at only one predetermined time point, and continuous monitoring of a single sample is not possible. Additionally, in contrast to [3H]-thymidine autoradiography, BrdU immunohistochemistry is not stoichiometric (R. S, Nowakowski et al., J. Neurocytol., 1989, 18: 311-318; R. S, Nowakowski and N. L. Hayes, Science, 2000, 288: 771). Thus, the intensity or extent of labeling is highly dependent on the conditions used for detection and does not necessarily reflect the magnitude of DNA replication. For this reason, BrdU labeling as a measure of cell division is especially vulnerable to misinterpretation (P. Rakic, Nature Rev. Neurosci., 2002, 3: 56-71).
More recently, a stable isotope-mass spectrometric technique has been developed that resolves some of the problems associated with the [3H]-thymidine and BrdU incorporation methods (D. C. Macallan et al., Proc. Natl. Acad. Sci. USA, 1998, 95: 708-713; M. K. Hellerstein, Immunol. Today, 1999, 20: 438-441; M. Hellerstein et al., Nature Med., 1999, 5: 83-89; J. M. McCune et al., J. Clin. Invest., 2000, 105: R1-8; H. Mohri et al., J. Exp. Med., 2001, 94: 1277-1288; R. M. Ribeiro et al., Proc. Natl. Acad. Sci. USA, 2002, 99: 15572-15577, R. M. Ribeiro et al., Bull. Math. Biol., 2002, 64: 385-405). In this technique, the deoxyribose moiety of nucleotides in replicating DNA is labeled endogenously, through the de novo nucleotide synthesis pathway by using stable isotope 2H- or 13C-labeled glucose. The isotopic enrichment of the DNA is then detected and quantified by gas chromatographic/mass spectrometric (GC/MS) analysis after isolation, denaturation and hydrolysis of genomic DNA and TMS (trimethylsyl) derivatization of the resulting deoxyribonucleosides. Although this method has several advantages including being safe for use in humans, it has disadvantages including that it involves a lengthy and destructive processing of the sample prior to detection.
Clearly, improved nucleic acid labeling techniques are still needed for the study of cell cycle kinetics, DNA synthesis and cellular proliferation in vitro and in vivo. In particular, the development of techniques that are simple, rapid, and sensitive and that do not require extensive sample preparation and/or do not result in sample destruction remains highly desirable.